Slim waveguide coupling apparatus and method

ABSTRACT

In various embodiments, an illumination structure includes a discrete light source disposed proximate a bottom surface of a waveguide and below a depression in a top surface thereof. A top mirror may be disposed above the discrete light source to convert modes of light emitted from the discrete light source into trapped modes, thereby increasing the coupling efficiency of the illumination structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/500,889, filed on Jul. 10, 2009, which claims priority toand the benefit of U.S. Provisional Patent Application Ser. No.61/079,582, filed on Jul. 10, 2008, and U.S. Provisional PatentApplication Ser. No. 61/206,080, filed on Jan. 27, 2009. Each of theseapplications is hereby incorporated herein by reference in its entirety.

TECHNICAL FIELD

Embodiments of the invention generally relate to illumination panels,and, more specifically, to coupling of light sources to waveguides.

BACKGROUND

Thin, planar illumination systems are desirable for many applicationssuch as, for example, low-profile back-illuminated displays. FIG. 1illustrates such an illumination system 100 fabricated by coupling adiscrete light source, e.g., a light-emitting diode (“LED”) 102 to anarrow face 104 of a waveguide 106. Generally, a waveguide 106 having arefractive index of N=1.5 and an LED 102 having Lambertian outputcharacteristics, combined as in the illumination system 100, have atheoretical maximum coupling efficiency limit of 85%. That is, at most85% of the light emitted by the LED 102 will be trapped within thewaveguide 106, and the remaining portion of the emitted light will belost.

This coupling inefficiency may be attributed to the constraints inherentin the side-emitting LED design of the illumination system 100. Whilethinner waveguides are desirable, the thickness t of the waveguide mustbe larger than the width d of the LED in order to achieve couplingefficiencies approaching 85%. Relatively high coupling efficiencies(e.g., greater than approximately 70%) are difficult to obtain for caseswhere the thickness t of the waveguide is smaller than the width d ofthe LED. Thus, as waveguides become thinner, the coupling efficiency ofthe waveguide decreases and more light is lost. The couplinginefficiency may even set a practical lower bound on the thickness ofthe waveguide 106. In addition, many side-emitting illumination systemsutilize specially engineered LED and waveguide structures in order toincrease the coupling efficiency. These structures not only add to thecomplexity and cost of the illumination system 100 but also increase itsthickness.

Therefore, there is a need for systems and methods of coupling LEDs towaveguides in which high coupling efficiencies are obtained, whileremaining easily and inexpensively manufacturable.

SUMMARY

The present invention enables superior coupling efficiencies betweenlight sources such as LEDs and thin waveguides by utilizing an LED thatis embedded inside a waveguide and emits all its light inside thewaveguide. A first portion of the emitted light propagates through thewaveguide because its emission angle (with respect to the waveguide'supper surface) results in total internal reflection (“TIR”) of the firstportion. A second portion of the light is not emitted at an angle to thewaveguide's upper (and/or lower) surface resulting in total internalreflection; this second portion may be reflected by a specular mirrorpositioned above the LED. The light thus reflected also undergoes totalinternal reflection, improving the coupling efficiency to the waveguide.Embodiments of the invention enable the waveguide to have a smallthickness, e.g., less than approximately 1 mm, with an LED having awidth of approximately 1 mm. Moreover, embodiments of the invention alsoutilize standard waveguide shapes and standard LED light sources notengineered to re-reflect emitted light, thereby reducing the cost andcomplexity of the system. Finally, embodiments of the invention do notrequire specially designed reflectors or selectively coated waveguidesurfaces, further decreasing manufacturing cost and complexity.

Advantages of the invention include the ability to use top-emitting(e.g., Lambertian) LEDs instead of side-emitting LEDs, which enables theuse of inexpensive and high-power top-emitting bare-die LED chips thatcan emit light in all directions. Such chips may be placed below thewaveguide instead of attached to a narrow side of the waveguide. Amirror may be used that exhibits specular reflection instead of a mirrorexhibiting total internal reflection or a diffuser (i.e., a surfaceexhibiting diffuse reflection). The mirror may be positioned anddesigned such that most of the light emitted from the LED, e.g., morethan approximately 85%, is coupled to the waveguide. Moreover, the lightreflected by the mirror may be within the propagation angle of thewaveguide after reflection. Back-reflection of light toward the LED maybe prevented, thereby obviating the need for specially engineeredincreased reflectivity of the LED surface (or the surface of the LEDelectrode) to decrease light absorption by the LED.

The waveguide and LED may be included in a full illumination devicefeaturing in-coupling, concentration, propagation, and out-couplingregions. Light propagating inside the waveguide in a direction away fromthe out-coupling region may be redirected toward the out-coupling regionby a specially engineered shape of the waveguide's back edge. A topmirror may be included to reduce or prevent reflection of light backtoward the LED in the vertical direction; a concentrating mirror mayalso be included (on, e.g., the back surface of the waveguide) to reduceor prevent reflection of light back toward the LED in the horizontaldirection.

The full illumination device may provide efficient (e.g., greater thanapproximately 70% or even 85% or more) light in-coupling to a thinwaveguide, even when the thickness of the waveguide is approximatelyequal to (or even less than) the LED width.

In an aspect, embodiments of the invention feature an illuminationstructure including or consisting essentially of a waveguide, a discretelight source, and a top mirror. The waveguide has a depression in itstop surface, the discrete light source is disposed proximate the bottomsurface of the waveguide and below the depression, and the top mirror isdisposed above the discrete light source.

The top mirror may include or consist essentially of a conical mirrordisposed over and at lest substantially filling the depression. An airgap may be disposed between the conical mirror and the depression. Thetop mirror may include or consist essentially of a substantially flatmirror disposed over substantially all of the depression. Thesubstantially flat mirror may be a specular mirror, a diffusivereflector, a Fresnel reflector, and/or a diffractive optical element. Atleast a portion of light emitted from the discrete light source may bereflected from a side surface of the waveguide through the depression ina confined mode of the waveguide. At least a portion of light emittedfrom the discrete light source may be reflected from a side surface ofthe waveguide through the depression and reflected back into thewaveguide in a confined mode of the waveguide by the substantially flatmirror.

In another aspect, embodiments of the invention feature an illuminationstructure including or consisting essentially of a waveguide having acavity through a thickness thereof, a discrete light source disposed inthe cavity, and a top mirror disposed in the cavity above the discretelight source. The top mirror may include or consist essentially of aconical mirror and/or a substantially flat mirror. The portion of thecavity not occupied by the top mirror and the dicrete light source maybe filled with an index-matching material. The index-matching materialmay have an index of refraction substantially matching that of thewaveguide. The cross-sectional area of the cavity proximate the topmirror may be larger than the cross-sectional area of the cavityproximate the discrete light source.

In yet another aspect, embodiments of the invention feature a method forcoupling light emitted from a discrete light source to a waveguide.Light is emitted from a discrete light source disposed within awaveguide. A portion of the emitted light is reflected from a top mirrordisposed above a depression in the top surface of the waveguide (thedepression being disposed above the discrete light source), so as toconfine the reflected portion of the emitted light within the waveguide.

The top mirror may include or consist essentially of a conical mirrorsubstantially filling the depression, and the portion of the emittedlight may traverse an air gap between the depression and the top mirrorbefore being reflected. A second portion of the emitted light may bereflected from a side surface of the waveguide through the depression soas to confine the reflected second portion of the emitted light withinthe waveguide. The reflected second portion of the emitted light may notstrike the top mirror after being reflected through the depression. Thereflected second portion of the emitted light may strike the top mirrorafter being reflected through the depression, and the reflection fromthe top mirror may confine the reflected second portion of the emittedlight within the waveguide.

In a further aspect, embodiments of the invention feature a method forcoupling light emitted from a discrete light source to a waveguide.Light is emitted from a discrete light source disposed within a cavityextending through the thickness of a waveguide. A portion of the emittedlight is reflected from a top mirror disposed above at least a portionof the cavity so as to confine the reflected portion of the emittedlight within the waveguide. The top mirror may include or consistessentially of a conical mirror and/or a substantially flat mirror. Theportion of the cavity not occupied by the top mirror and the dicretelight source may be filled with an index-matching material. Theindex-matching material may have an index of refraction substantiallymatching that of the waveguide.

In another aspect, embodiments of the invention feature a method offorming an illumination structure. A substantially planar waveguidehaving input and output regions is formed. A depression in the topsurface of the waveguide in the input region and a notch in the bottomsurface of the waveguide in the input region are formed. A top mirror isdisposed over the depression, and a discrete light source is disposedwithin the notch. The top mirror may include or consist essentially of aconical mirror, and disposing the top mirror over the depression mayinclude substantially filling the depression with the top mirror. An airgap may be left between the top mirror and the depression. The topmirror may include or consist essentially of a substantially flatmirror, and disposing the top mirror over the depression may includesubstantially covering the depression with the top mirror. At least aportion of the notch may be filled with an index-matching material. Theindex-matching material may have an index of refraction substantiallymatching that of the waveguide.

These and other objects, along with advantages and features of thepresent invention herein disclosed, will become more apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and mayexist in various combinations and permutations.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. In the following description,various embodiments of the present invention are described withreference to the following drawings, in which:

FIG. 1 is a cross-sectional view of a prior-art side-mountedillumination system;

FIG. 2 is a cross-sectional view of an illumination system featuring aparabolic mirror;

FIG. 3 is a cross-sectional view of an illumination system showingrelative dimensions;

FIG. 4 is a cross-sectional view of an illumination system with anembedded LED;

FIG. 5 is a cross-sectional view of an illumination system featuring anLED sub-assembly;

FIG. 6 is a cross-sectional view of an illumination system featuring abottom diffusive mirror;

FIG. 7 is a cross-sectional view of an illumination system featuring analternative mirror design;

FIG. 8 is a plan view of an illumination panel for use in a tiledconfiguration of panels;

FIG. 9 is a plan view of an in-coupling region featuring ellipticalmirror sections;

FIG. 10 is a plan view of an illumination panel featuring multiple LEDs;

FIG. 11 is a plan view of an illumination panel featuring multiple LEDsin an alternate configuration;

FIG. 12 is a cross-sectional view of an illumination panel;

FIG. 13 is a plan view of an illumination panel featuring scatteringstructures;

FIG. 14 is a cross-sectional view of an illumination panel featuring twolayers;

FIGS. 15A and 15B are a perspective view and a plan view, respectively,of an illumination panel featuring multiple LEDs;

FIG. 16 is a cross-sectional view of an illumination panel featuring anasymmetrically placed LED;

FIG. 17 is a cross-sectional view of an illumination panel featuring aphosphor layer;

FIGS. 18A and 18B are a cross-sectional view and top view, respectively,of an illumination panel;

FIG. 19 is a cross-sectional view of an illumination system having acavity and a curved mirror inserted therein;

FIG. 20 is a cross-sectional view of an illumination system having awaveguide with a depression in a surface thereof and a curved mirrordisposed over the depression; and

FIGS. 21A and 21B are cross-sectional views of an illumination systemhaving a waveguide with a depression in a surface thereof and asubstantially flat mirror disposed over the depression.

DETAILED DESCRIPTION

Described herein are methods and systems for coupling light from a lightsource to a waveguide. Embodiments of the invention apply to twodifferent cases: (1) when a thickness, t, of the waveguide isapproximately equal to or larger than the width, d, of the light sourceand (2) when t is less than d. In cases where t is approximately equalto or larger than d, a curved, semi-curved, broken-line, or single-linetop mirror redirects light that strikes it into confined modes (i.e.,propagation paths confined by the waveguide). As utilized herein, withreference to the two-dimensional cross-sectional view of the mirrorelement, “curved” refers to a mirror with a curved shape, “semi-curved”refers to a mirror with a curved segment and straight segments,“broken-line” refers to a mirror having several discrete straightsegments that approximate a curved segment, and “single-line” refers toa mirror consisting of a straight segment. The mirror is positioned suchthat light propagating at an angle less than necessary for TIR (andwhich therefore will not be confined within the waveguide) strikes themirror. Thus, the mirror shape and position may enable the redirectionof light from unconfined modes into confined modes. Any light already ina confined mode that does strike the mirror may remain in a confinedmode after reflection.

FIG. 2 illustrates one example of a parabolic mirror 202 that is setaround a point A (the top-right corner 204 of an LED 206), such that thelight rays emitted from the point A toward the region of the mirrorbounded by points M and D (“region M-D”) are reflected back into thewaveguide 208 at an angle equal to (or larger than) the critical angleθ_(T) for total internal reflection. Thus, all of the light rays fromthe portion of the top surface 210 of the LED 206 bounded by points Aand B (“region A-B”) may be reflected at angles larger than θ_(T) so asto be confined. The light rays from the region bounded by points B and C(“region B-C”) may undergo one or multiple reflections at the mirrorsuch that their final propagating angle is also greater than θ_(T).

Light rays that do not strike the mirror are, by definition, alreadyconfined (i.e., propagating at an angle greater than θ_(T)) because thelight ray from point A to point M and the light ray from point A topoint D propagate at an angle θ_(T). The light ray from point A to pointM propagates perpendicularly to the surface of the mirror 202 at point Msuch that the light is reflected back toward point A. At point D, themirror 202 is substantially parallel to the plane of the waveguide 208,and the light ray propagating from point A at an angle θ_(T) isreflected into the waveguide 208 at an angle θ_(T) (pursuant to Snell'sLaw). The light rays may act similarly if one or more cladding layers(not shown) are added to the waveguide 208.

FIG. 3 depicts an exemplary embodiment of the invention that includesrelative dimensions. As shown in FIG. 3, the waveguide 208 thickness t(as measured from the top surface 210 of the LED 206 to the top surface302 of the waveguide 208) may be approximately equal to the LED 206width d.

In further embodiments, the mirror configuration takes alignmenttolerances into account. With reference to FIG. 3, the mirror 304 may bea parabolic mirror defined by the equation y=ax², where point M definesthe origin point (i.e., x=0 and y=0), point A lies on the y-axis, anda=¼_(y) ₀ . The width d of the LED 206 and the thickness t of thewaveguide 208 may thus be defined as √{square root over (2)}y₀, and theshortest distance between the mirror 304 and the surface 210 (at point Mof the mirror 304) is y₀/√{square root over (2)}. Because point M of themirror 304 is defined with reference to point A (the right edge of theLED 206), slight misalignment of the LED 206 may lead to less efficientlight coupling. The sensitivity to such misalignment may be reduced bypositioning point M with respect to a point A′ offset from point A by atypical misalignment tolerance, e.g., approximately 0.1 mm.

Referring to FIG. 4, in various embodiments, light emitted by or throughside faces 412 of the LED 206 may be coupled into the waveguide 208. Insuch cases, the LED 206 may be embedded within the waveguide 208, asshown, which then may have a thickness of approximately t+h (i.e.,slightly larger than the LED width d). In this configuration, the lightfrom an LED 206 having a width d of approximately 1 mm may be coupled toa waveguide 208 having a thickness of approximately 1 mm (or slightlygreater than approximately 1 mm) while achieving a coupling efficiencygreater than approximately 85%, or even greater than approximately 95%.

Light emitted from a side surface 412 of the LED 206 (assuming the LED206 is capable of emitting such light) may propagate at angles less thanthe angle required for total internal reflection. A reflecting layer 402may therefore be placed at the bottom facet 404 of the waveguide 208, atleast spanning and desirably extending beyond the perimeter of the LED206, to capture the unconfined light. The reflecting layer 402 mayreflect unconfined light toward the top mirror 304. In that case, theposition of point D may be determined by the ray 416 from point S′ thatstrikes the waveguide 208 top surface at an incident angle equal to thecritical angle θ_(T.). The distance between points S and S′ may beapproximately equal to the thickness h of the LED 206.

In accordance with embodiments of the invention, the non-zero thicknessh of the LED 206 is considered when designing the shape and placement ofthe mirror 202. This consideration is important for LEDs that not onlyemit light from their top surfaces 210 (as in the case described above),but also from their side surfaces 412. The light emitted from or throughthe side surfaces 412 of the LED 206 may also be confined in thewaveguide 208. Referring to FIG. 2, the critical angle θ_(T) isdetermined by the ray from point S (the bottom-right corner 414 of theLED 206) to point D. Thus, θ_(T) will be slightly larger than the θ_(T)described above (which may apply to an LED that emits light only fromits top surface). This selection of the proper θ_(T) enables thecoupling of substantially all light emitted from the LED 206.

For example, the critical angle for total internal reflection for awaveguide, cladded by air (N_(clad)=1), with a refractive index of N=1.5is approximately 42 degrees. However, for an LED 206 having a thicknessh of approximately 0.1 mm and a waveguide 208 having a thickness t ofapproximately 1 mm, the critical angle θ_(T) (measured from point A) isapproximately 45 degrees. In general, the critical angle θ_(T) for TIRfor a waveguide clad in a material having an index of refractionN_(clad) (such as, for example, a waveguide core surrounded by claddinglayers) is given by θ_(T)=sin⁻¹(N_(clad)/N).

FIG. 5 illustrates an LED light source 206 assembled on a top surface ofan LED sub-assembly module 502, such as a printed-circuit board (“PCB”)or a carrier plate, which provides a mechanical interface to hold theLED light source 206 in position and/or an electrical interface tooperate the LED light source 206. In these assemblies, an index-matchingmaterial 504 may be used to fill the space between the LED 206 and thetop mirror 304 (and/or bottom facet 404), thereby providing an opticalconnection not only between the LED 206 and the top mirror 304 (and/orbottom facet 404) but also between the top mirror 304 (and/or bottomfacet 404) and the top surface 506 of the LED sub-assembly 502.

The top surface 506 of the LED sub-assembly 502 may extend beyond thedimensions of the LED light source 206, thus allowing guided light raysto reach the top surface 506 of the LED sub-assembly 502. Thereflectivity quality of the LED sub-assembly surface 506 may not becontrollable and is less than the TIR reflectivity. Therefore, the topmirror 304 positioned over the LED light source 206 is preferablydesigned to reflect light away from the LED sub-assembly 502. In oneembodiment, as shown in FIG. 5, the top mirror 304 has a paraboliccontour.

Referring again to FIG. 5, a light ray 508 is coupled from a point S atthe right edge 510 of the LED light source 206, reflected from a point Mon the top mirror 304, and radiates back along a path 510 to a point S″at the end of the index-matching region 504 of the LED sub-assembly 502.Other rays emitted from the LED light source may be reflected past theLED sub-assembly 502 to the region beyond point S″. Point S′, which isapproximately halfway between points S and S″, may be used as areference point for forming the parabolic shape of the top mirror 504.For example, a light ray 512 emitted from point S′ that strikes the topmirror at point M may be reflected back toward point S′. The shape ofthe top mirror 304 at point M may a sharp edge or a curve. For example,if the width of the top mirror 304 is 2 mm, the shape of the top mirror304 at point M may be a curve having a radius of 0.1 mm. Such a shapemay decrease the manufacturing cost and/or complexity of the top mirror304 relative to the cost and complexity of manufacturing a sharp edge atpoint M without significantly affecting the performance of the topmirror 304.

In alternative embodiments where t is less than d, in general, all theunconfined light rays (propagating at angles below the critical angle)strike a curved, semi-curved, or broken-line top mirror that redirectsthe light back into the waveguide. The mirror preferably does notreflect the light back into the LED. While most of these reflected raysare redirected to confined modes (i.e., propagation paths confined bythe waveguide), some remain propagating at angles below the criticalangle (i.e., they remain unconfined modes). These unconfined modes maybe redirected toward a bottom diffusive reflector which redirects atleast a significant portion (for example, greater than 55%) of thisremaining light into confined modes.

FIG. 6 depicts a top specular mirror 602 including a curved section 604between points M and D and a flat section 606 between points D and E.The flat section 606 may be substantially parallel to a top surface 608of the LED 206. A bottom diffusive mirror 610 extends from approximatelythe LED 206 edge A to a point G and may be designed so that most or allof the reflected light already corresponding to confined modes does notstrike the bottom diffusive mirror 610. A similar bottom diffusivemirror may be disposed on the other side of the LED 206. Light emittedfrom the LED 206 that does not strike the curved section 604 may insteadstrike the flat section 606 and be reflected toward the bottom diffusivemirror 610. This light, initially in an unconfined mode, strikes thediffusive mirror 610 and is reflected into confined modes. Couplingefficiencies greater than approximately 80% may be obtained with thisconfiguration.

FIG. 7 depicts an embodiment in which the curved section 604 of themirror 602 has a radius approximately equal to half the width of the LED206 (i.e., d/2), and point M of the mirror approximately coincides withpoint B (the center of the top surface 608 of the LED 206). Thewaveguide 208 thickness is approximately equal to t+h, where t=d/2 and his the LED thickness. Light rays from point A that propagate toward thecurved section 604 are reflected back toward point A. Light rays frompoint B propagating at approximately 45 degrees strike the top mirror602 at point D and are reflected to point G of the diffusive mirror 610.Thus, most or all of the unconfined modes emitted from the LED 206between points A and B strike the diffusive mirror 610 and are reflectedinto confined modes.

In one embodiment, large illumination structures are formed by arranging(or “tiling”) panels that include the above-described waveguidestructures. In a tiled structure, each panel may include or consistessentially of an input region and an output region, and the outputregion of one panel may cover the input region of an adjoining panel.Thus, only output regions may be observable from above the tiledstructure. In an alternate embodiment, a large illumination structure isformed by placing panels adjoining each other (i.e., in a non-tiledconfiguration with no overlap between panels) such that light is coupledout from the entire panel surface.

FIG. 8 illustrates a panel 802 for use in a tiled configuration of oneor more panels. The output region 804 of each panel 802 may includescattering structures (such as hemispheres, wedges, particles, and/orother similar structures). Light from an LED 806 disposed in an inputregion 808 is preferably directed toward the output region 804 such thatthe light does not pass through the LED 806 and/or a top mirror. Lightrays 810 emitted by the LED 806 away from the output region 804 may bereflected back toward the output region 804 by a back horizontal mirror812. In a preferred embodiment, the back horizontal mirror 812 is notperfectly linear, but rather includes or consists essentially of one ortwo elliptical mirror sections.

FIG. 9 shows, in one embodiment, elliptical mirror sections 906, 908each define a portion of the back horizontal mirror 812. The LED 806 maybe positioned approximately at a position corresponding to the polescommon to each ellipse 906, 906 (which also have poles 902, 904). Thus,substantially all of the light rays emitted from the LED 806 may beredirected (and distributed) to the output coupling region 804 while notpassing through the LED 806.

In some embodiments, emission of white light (e.g., formed by thecombination of red, green, and blue (“RGB”) light) or lightcorresponding to combinations of red, green, and blue light isdesirable. In such embodiments, each single LED of the above-describedembodiments may be replaced by a set of at least three LEDs: at leastone emitting red light, at least one emitting green light, and at leastone emitting blue light.

FIG. 10 illustrates an embodiment in which a plurality of LEDs 1002 are“crowded” (i.e., arranged close together, but not necessarilycollinearly), such that color mixing is optimized and the loss due tolight propagating from one LED directly to the other LEDs is minimal.The LEDs may include a red LED 1004, a green LED 1006, and a blue LED1008. In another embodiment, shown in FIG. 11, separate horizontal backmirrors 1102, 1104, 1106 are provided for each LED 1004, 1006, 1008,respectively, and the colors are mixed to white light while propagatingin the input region (i.e., before the light reaches the output region804).

FIG. 12 illustrates a side view of a panel 1202 including an LED 1204disposed on a substrate 1206. In an “isolated” or “non-tiled” panelconfiguration, light is preferably emitted from the entire top surface1208 of the panel 1202, including the region 1210 above the top curvedmirror 1212. In one embodiment, the intensity of the light emittedthrough the top curved mirror 1212 is equal to the intensity of thelight coupled out from the rest of the top surface 1208 of the panel1202. A suitable absorber 1214 may be placed above the top curved mirror1212 to emit light of a desired intensity therefrom.

Some light may penetrate through the top curved mirror 1212. Forexample, suppose the LED 1204 has a width of 0.5 mm, the area of the topcurved mirror 1212 is 1.5²=2.25 mm² (in accordance with the mirror 304of FIG. 3, above). Assuming a panel 1202 of width 10 cm and depth 10 cm,100% output coupling efficiency, and a mirror transparency of 1%, inorder to obtain the same intensity across the entire panel, the absorber1214 should absorb ˜98% of the light intensity.

In one embodiment, the absorber 1214 is diffusive. In anotherembodiment, scattering structures 1216 may be placed across some or allof a top surface 1208 of the panel 1202 to aid in the out-coupling oflight. A mirror 1218 may placed at the bottom surface 1220 of the panel1202.

Several conditions may aid the incorporation of RGB LEDs into anisolated panel configuration. First, the LEDs may be crowded (i.e.,positioned closely together) to permit the use of a single out-couplingstructure for all of the LEDs. In order to maintain a substantiallyuniform light level emitted across the panel, the density of scatteringstructures preferably increases as a function of distance away from theLEDs. Alternatively, scattering structures with increasing scatteringcoefficients (as a function of distance away from the LEDs) may beutilized. Preferably, the region above the top curved mirror of one LEDmay be transparent to light emitted by the other LEDs in order tofacilitate out-coupling of light of all colors. FIGS. 13 and 14 depict asuitable configuration.

FIG. 13 depicts a top view of an isolated illumination panel 1302 thatincludes four crowded LEDs 1304 (e.g., one red, two green, and oneblue—“RGGB”). Out-coupling scattering structures 1306 are providedbetween and/or around the LEDs 1304. A cross-sectional view of two ofthe LEDs 1304, including a red LED 1402 and a blue LED 1404, and theircorresponding upper curved mirrors 1406, 1408 is shown in FIG. 14. A toplayer 1410 of the waveguide disposed above the upper curved mirrors1406, 1408 includes scattering structures 1412 for facilitating theout-coupling of light in the regions above the upper curved mirrors1406, 1408. The top layer 1410 is preferably optically connected withthe bottom layer 1414 (the layer containing the curved mirrors 1406,1408 and the LEDs 1402, 1404) such that light freely propagates from onelayer to the other. The scattering structures 1412 may be disposed atthe top surface 1416 of the top layer 1410. In other embodiments, thescattering structures 1412 are incorporated in other portions of the toplayer 1410, or even in the bottom layer 1414.

FIG. 14 also depicts several different light rays, each traveling adifferent path from an LED 1402, 1404 to emission from the waveguide1418. Ray (a) is emitted from the blue LED 1404 and scattered from theblue top curved mirror 1408 into the waveguide 1418. Ray (b) is the partof ray (a) that penetrates through the mirror 1408 and is emitted fromthe waveguide 1418 by the scattering structures 1412. Ray (c) is emittedfrom the red LED 1402 and then from the waveguide 1418 through thescattering structures 1412. Ray (d) is the part of ray (c) that isreflected back to the waveguide 1418 and, after reflection therefrom, isemitted from the waveguide 1418 in the region above the blue top curvedmirror 1408.

In some embodiments featuring multiple LEDs, such as the RGB LEDsdescribed above, each LED has its own, separate top mirror. In theseembodiments, each top mirror is shaped like a cone, pyramid, or anyother non-flat shape suitable to retaining light within the waveguidethat would otherwise escape. In other embodiments, more than one of theLEDs share a single top mirror. The LEDs may be arranged in a line, andthe shared top mirror may be shaped like a prism with curved sidefacets. In one embodiment, as illustrated by FIGS. 15A and 15B, theshared top mirror 1502 is a triangular prism having triangular sidefacets and the array of LEDs 1504 includes RRGGB LEDs.

The top curved mirror 1502 is not limited to symmetric structures; itmay be designed asymmetrically if, for example, the LEDs 1504 areconfigured asymmetrically, such as an LED not located at the center ofthe LED sub-assembly 1506. In such a case, in order to avoid raysstriking the LED sub-assembly (as illustrated in FIG. 5), the top curvedmirror 1502 may be designed asymmetrically and/or located asymmetrically(relative to the center of the LEDs 1504). An example is shown in FIG.16; there, the LED sub-assembly 1602 is asymmetric (relative to thecenter of the LED 1604) and, accordingly, the top curved mirror 1606 isasymmetrically located (relative to the center of the LED 1604). Thecurved mirror 1606 is located such that rays from point A are reflectedback from point M toward point A and rays from point C are reflectedfrom point M toward point C′. In this embodiment, a virtual point C″ islocated approximately at the center between point C and point C′; i.e.,virtual rays from point C″ striking point M will be back reflected topoint C″. In this embodiment, point M is located at the center betweenpoints A and C″.

FIG. 17 illustrates a side view of a waveguide 1702 that features aphosphor layer 1704. The phosphor layer 1704 may produce white lightfrom a single-color LED light source 1706. The phosphor layer 1704preferably converts some of the light from the LED 1706 to anotherwavelength. The original light adds to the converted light, creatingwhite light. For example, a blue LED may be combined with a yellowphosphor layer, and the blue light from the LED may combine with theyellow light from the phosphor layer to produce white light. In someembodiments, phosphors are utilized to facilitate the emission of whitelight (or light of another preferred wavelength).

In accordance with embodiments of the invention, the configurationdepicted in FIG. 17 includes a patterned waveguide 1702 and an LED chip1716 with matching indices of refraction. The waveguide may be anoptical polymer, e.g., a polymethyl methacrylate (PMMA), and may includea bottom notch 1720 (for embedding the LED 1716 therein) and a topcurved mirror 1708. The waveguide 1702 may be formed by molding or byanother suitable process known in the art. The waveguide 1702 may alsoinclude a bottom mirror 1712. The LED chip 1716 may be mounted into thewaveguide notch 1720 such that it substantially seals the notch 1720along the bottom surface 1722 of the waveguide 1702. Any remaining spacein the notch 1720 may be filled with an index-matching material 1718having an index of refraction matching that of the waveguide 1702. Thephosphor layer 1704 may be formed above or around the LED chip 1716, orit may be deposited directly in the waveguide notch 1720 prior tointroduction of the LED chip 1716. An absorber layer 1710 and asubstrate 1714 may also be included.

FIGS. 18A and 18B depict a side and top view, respectively, of anisolated panel configuration 1800, including a phosphor layer, that addsscattering structures 1802. The illumination system 1800 may be utilizedin cases where the waveguide 1702 thickness is either approximatelyequal to (or even larger than) the width of the LED 1716, as well as incases where the waveguide thickness is less than the LED width. Thephosphor-containing system may also be utilized in tiled or isolatedconfigurations, as described above.

Selective “coatings” on particular areas of a waveguide that formmirrors and/or reflectors may be time consuming, expensive, or difficultto manufacture. Also, forming a coated surface with approximately 100%reflectivity is quite difficult and often impossible. Thus, severalembodiments of the present invention facilitate manufacturing byeliminating the need for selective-area coatings while maintaining highcoupling efficiencies.

FIG. 19 depicts a side view of a waveguide 1910 having, through at leasta substantial portion of its thickness, a cavity 1920 formed therein. AnLED 1930 (or other light source) may abut a bottom portion of cavity1920 or be embedded therein, as shown in FIG. 19. LED 1930 may besupported by a sub-assembly 1940 (as described above). A conical mirror1950 is positioned (e.g., inserted or deposited) within the top portionof cavity 1920 (i.e., opposing LED 1930). Conical mirror 1950 may becoated on its “front,” i.e., curved, surface. The remaining volume ofcavity 1920 is preferably filled with an index-matching material 1960that has an index of refraction substantially the same as that ofwaveguide 1910. Conical mirror 1950 is preferably three-dimensional(rather than, e.g., a shaped two-dimensional sheet) and substantiallyconical, parabolic, pyramidal, and/or prismatic in shape. Conical mirror1950 may include or consist essentially of a reflective metal, e.g.,aluminum, and may even have a reflective coating on at least the surfacepositioned within the cavity. In an alternate embodiment, conical mirror1950 includes or consists essentially of topaz or a polymer such as PMMAand is coated with a coating of a reflective metal, e.g., aluminumand/or silver. In embodiments in which conical mirror 1950 is coated,generally the surface of conical mirror 1950 facing the incident lightis coated and thus reflects the light. Since the coating may be formedon conical mirror 1950 through the use of an adhesive materialtherebetween, this configuration prevents incident light from having topass through or being unexpectedly reflected by the adhesive material.In some embodiments, a substantially flat mirror (as described belowwith reference to FIGS. 21A and 21B), rather than or in addition toconical mirror 1950, may be formed over or within cavity 1920.

Although the embodiment depicted in FIG. 19 may be easily manufactured,the optical coupling efficiency of the waveguide may be further improvedby increasing even further the proportion of light rays emitted from anLED that undergo total internal reflection within the waveguide.Referring to FIG. 20, a waveguide 2010 has an LED 1930 embedded in acavity (or notch) therewithin and a top depression 2015 similar in shapeto conical mirror 1950. Depression 2015 is an angled interior featurethat reflects impinging light, preferably into confined modes ofwaveguide 2010. In particular embodiments, depression 2015 isapproximately conical, parabolic, pyramidal, or prismatic in shape. Asin FIG. 19, an index-matching material 1960 may surround LED 1930 in thecavity. Conical mirror 1950 is positioned in depression 2015 ofwaveguide 2010, leaving a small air gap 2020. Air gap 2020 may have athickness ranging from approximately 100 μm to approximately 200 μm, oreven smaller (e.g., ranging from approximately 10 μm to approximately 50μm, or may even be less than approximately 10 μm). Air gap 2020effectively acts as a “cladding layer” for depression 2015 of waveguide2010, increasing the amount of light from LED 1930 that undergoes totalinternal reflection into waveguide 2010. In certain embodiments, air gap2020 is at least partially filled with a filler material that may have alower refractive index than the refractive index of the waveguidematerial and/or adhesive properties for adhering conical mirror 1950into depression 2015 of waveguide 2010. The difference in refractiveindices between the filler material and the waveguide material may beless than approximately 0.2. For example, the waveguide material may bePMMA having a refractive index of approximately 1.5 and the fillermaterial may be silicone having a refractive index of approximately 1.3.In various embodiments, a portion of the light emitted by LED 1930strikes depression 2015 at an angle such that it “escapes” the waveguide(i.e., transmits through the surface of the waveguide at depression2015). The escape angle may be defined by Snell's Law, and depends uponthe indices of refraction of the waveguide and of air gap 2020 (or acladding material therewithin). Any light that escapes depression 2015of waveguide 2010 is substantially reflected, by conical mirror 1950,into waveguide 2010 where it may undergo total internal reflection. Thusthe optical coupling efficiency of the waveguide is enhanced without theneed for a cavity through the entire waveguide thickness.

The high optical coupling efficiencies of the embodiments depicted inFIGS. 19 and 20 are demonstrated by the following examples. Assumingthat the sides of the waveguide are coated and exhibit 98% reflectivity,and that the conical mirror also exhibits 98% reflectivity, theembodiments depicted in FIGS. 19 and 20 exhibit approximately 94.5% and92% optical coupling efficiency, respectively. If the conical mirror hasa reflectivity of only 91%, then the embodiments exhibit respectiveoptical coupling efficiencies of approximately 89% and 91%. Thus, for aless reflective conical mirror, the addition of an air gap results ingreater light confinement in the waveguide.

In some embodiments, it is desirable to “route” light emitted from alight source (e.g., any of the above-described LEDs) through thewaveguide in a particular direction to a specific output region. Thus,an edge of the waveguide opposite this output region will generally bereflective in order to guide light travelling in the “wrong” directionback toward the output region. In such embodiments, it may be desirableto have light reflected from a side edge of a waveguide (and already ina total-internal-reflection mode) simply pass through a depression inthe waveguide surface rather than be reflected back in the “wrong”direction. However, some embodiments of the above-described conicalmirrors may prevent such propagation. In such embodiments, the conicalmirror may be replaced by a substantially flat mirror (i.e., one that issheet-like with minimal thickness) that covers (or “seals”) the openingto the depression in the waveguide. An additional advantage of thisembodiment is that it obviates the need for conical mirrors shaped tofit a waveguide depression, or vice versa.

FIG. 21A depicts such a variant, in which waveguide 2110 has abovedepression 2015 a substantially flat mirror 2120; the region betweendepression 2015 and flat mirror 2120 is “empty,” e.g., filled with onlyair. Flat mirror 2120 may be a specular mirror, a diffusive reflector, aFresnel reflector, a diffractive optical element, or other means ofreflecting light back into the waveguide. Flat mirror 2120 may be asubstantially planar sheet of reflective material, e.g., a foil such asVIKUITI Enhanced Specular Reflector Film, available from 3M of St. Paul,Minn. An advantage of this embodiment is depicted in FIG. 21B, whichillustrates three exemplary light rays emanating from LED 1930. Thelight ray labeled (a) is emitted from LED 1930 toward the side facet2130 of waveguide 2110 and reflected back toward depression 2015 at aconfined angle. Light ray (a) passes through the surfaces of depression2015, with a slight change in propagation angle, and proceeds throughthe waveguide in a confined mode. Such light would not be as efficientlycoupled in the waveguide had a conical mirror been in place indepression 2015.

In contrast, the light rays labeled (b) and (c) are, respectively,emitted directly toward depression 2015 or reflected back towarddepression 2015 in an unconfined mode. These light rays strike flatmirror 2120 (depicted as a diffusive mirror in FIG. 21B) and are largelyreflected back into the waveguide in confined modes (much as in theembodiments having a curved mirror described above). Effectively, theregion of waveguide 2110 covered by flat mirror 2120 receives a portionof the light from one or more light sources disposed thereunder (e.g.,LED 1930) and distributes it into the waveguide in confined modes. Flatmirror 2110 may have a reflectivity of greater than approximately 80%,greater than approximately 90%, or even greater than approximately 95%.

In an exemplary embodiment, flat mirror 2120 is a diffusive mirrorhaving a Lambertian distribution and 98% reflectivity (and the waveguideedges also have 98% reflectivity, as in the above examples). In such acase, an optical coupling efficiency of approximately 88% is obtained,very close to the values obtained with the conical mirrors. Thus, nearlythe same optical coupling efficiencies may be obtained in a much moremanufacturable fashion. For example, during fabrication of a waveguide,the above-described depression(s) may simply be embossed or “punched”into the waveguide material during a single-step process. Thedepressions may then simply be covered with flat mirrors, obviating theneed for conical mirrors, prisms, or the like.

The embodiments of FIGS. 19-21 may be utilized with other elementsdescribed above, e.g., phosphor layers, absorbers, scatteringstructures, and/or various panel configurations, and may be utilizedwith multiple light sources. The waveguides described herein may havethicknesses of approximately 1 mm, or even less.

The terms and expressions employed herein are used as terms andexpressions of description and not of limitation, and there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described or portions thereof. Inaddition, having described certain embodiments of the invention, it willbe apparent to those of ordinary skill in the art that other embodimentsincorporating the concepts disclosed herein may be used withoutdeparting from the spirit and scope of the invention. Accordingly, thedescribed embodiments are to be considered in all respects as onlyillustrative and not restrictive.

1. An illumination structure comprising: a waveguide having a depressionin a top surface thereof; a discrete light source disposed proximate abottom surface of the waveguide and below the depression; and a topmirror disposed above the discrete light source.
 2. The illuminationstructure of claim 1, wherein the top mirror comprises a conical mirrordisposed over and at least substantially filling the depression.
 3. Theillumination structure of claim 2, further comprising an air gapdisposed between the conical mirror and the depression.
 4. Theillumination structure of claim 1, wherein the top mirror comprises asubstantially flat mirror disposed over substantially all of thedepression.
 5. The illumination structure of claim 4, wherein thesubstantially flat mirror comprises at least one of a specular mirror, adiffusive reflector, a Fresnel reflector, or a diffractive opticalelement.
 6. The illumination structure of claim 4, wherein at least aportion of light emitted from the discrete light source is reflectedfrom a side surface of the waveguide through the depression in aconfined mode of the waveguide.
 7. The illumination structure of claim4, wherein at least a portion of light emitted from the discrete lightsource is reflected from a side surface of the waveguide through thedepression and reflected back into the waveguide in a confined mode ofthe waveguide by the substantially flat mirror.
 8. A method for couplinglight emitted from a discrete light source to a waveguide, the methodcomprising: emitting light from a discrete light source disposed withina waveguide; and reflecting a portion of the emitted light from a topmirror disposed above a depression in a top surface of the waveguide,the depression being disposed above the discrete light source, so as toconfine the reflected portion of the emitted light within the waveguide.9. The method of claim 8, wherein the top mirror comprises a conicalmirror substantially filling the depression, and the portion of theemitted light traverses an air gap between the depression and the topmirror before being reflected.
 10. The method of claim 8, furthercomprising reflecting a second portion of the emitted light from a sidesurface of the waveguide through the depression so as to confine thereflected second portion of the emitted light within the waveguide. 11.The method of claim 10, wherein the reflected second portion of theemitted light does not strike the top mirror after being reflectedthrough the depression.
 12. The method of claim 10, wherein thereflected second portion of the emitted light strikes the top mirrorafter being reflected through the depression, the reflection from thetop mirror confining the reflected second portion of the emitted lightwithin the waveguide.